UMTS Terrestrial Radio Access (UTRA) is the radio access network for the Universal Mobile Telecommunication System (UMTS), a 3G wireless communication system. The 3GPP standards body has approved a work item for Dual Cell High-Speed Downlink Packet Access (HSDPA) operation. The dual cell HSDPA operation can improve the bit-rate in terms of peak rate and average bit rate from the end-user's perspective. In dual cell HSDPA, separate data streams are modulated onto separate carriers, and transmitted together to User Equipment (UE) in the cell. The main focus of the study is on adjacent carriers. The aggregation of data in multiple carrier operation for Long Term Evolution (LTE) is not fully developed in 3GPP; however, the problems identified and solved by the present invention should, in general, apply to any wireless standard, such as LTE and GSM.
HSDPA in legacy UMTS systems is based on single cell, or carrier, operation. The frequency error between a transmitter and receiver—caused by frequency error in the base station and Doppler shift in channel—is compensated in the UE for single carrier operation. The required base station frequency accuracy for single-carrier in 3GPP is 0.05 ppm (for macro base stations). Considering two individual carriers for dual cell HSDPA, the relative frequency error between the carriers at the base station reference points could be up to 0.1 ppm. Currently there is no 3GPP requirement on base station relative frequency error; however, it can be assumed that a base station which uses the same source for frequency generation can have lower relative frequency error than 0.1 ppm since the error in the source can be excluded from the relative frequency error.
In dual cell HSDPA operation, data is modulated onto two carriers, which can advantageously be received in a multi-carrier receiver in the UE. For dual cell HSDPA, the concept of anchor and supplementary carrier is under discussion in 3GPP. The anchor carrier is contemplated as a legacy HSDPA carrier, which carries the same physical channels as a legacy HSDPA carrier. The supplementary carrier may only be required to carry a reduced set of physical channels, in order to reduce the control signaling overhead.
A number of multi-carrier receiver architectures are possible. One solution with high bandwidth and low complexity (and hence cost) utilizes multiple antennas and corresponding receiver paths in the analog front-end. The signals are then digitized, and the antenna/carrier separation, demodulation, decoding, and the like are performed in the digital domain, such as by software executing on Digital Signal Processors (DSP). For adjacent or sufficiently close carriers, a single local oscillator can be used to down-convert both carrier signals to baseband. The local oscillator frequency can be adjusted to correct for frequency errors between the transmitter and receiver. However, only one of the dual (or more) carriers may be frequency-corrected in this manner. Alternatively, the receiver may use a separate local oscillator for each carrier, and separately control each oscillator to frequency-correct each carrier. However, this approach adds cost and complexity.